1. Field of the Invention
The present invention relates to an apparatus and method for recording/reproducing optical information, and in particular, to an apparatus and method for recording/reproducing optical information that corrects spherical aberration generated by an error in the thickness of a substrate of an optical recording medium, in accordance with reproduced signal quality.
2. Related Background Art
In order to record a high-definition TV, a BD (Blu-ray Disk) apparatus has in recent years been proposed which is designed to increase density and capacity compared to the prior art apparatuses. In the BD apparatus, a semiconductor laser has a shorter oscillation wavelength, and an objective used has a higher NA.
In general, in a case where there is an error with respect to a designed value for the thickness of a substrate (light-transmissive layer) of an optical disk, spherical aberration on the optical recording medium is in proportion to the fourth power of the NA of the objective and is in inverse proportion to the wavelength. It is thus known that an error in the thickness of the substrate reduces a spot quality on the optical recording medium to degrade the recording/reproducing performance. Accordingly, because a BD apparatus using a shorter wavelength and a higher NA is more likely to generate spherical aberration than DVD apparatuses or the like, the spherical aberration must be corrected, so that apparatuses that correct the spherical aberration have been proposed. Further, techniques to use a plurality of data recording surfaces, for example, two layers have also been examined, and it is also necessary to correct spherical aberration in order to condense a light spot on two layers with different cover thicknesses without spherical aberration.
A technique for correcting spherical aberration is disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-233917 that detects an index such as amplitude, jitter or the like of a reproduced signal that indicates the quality of the reproduced signal to control a spherical aberration correcting element.
However, with an increase in recording density in the recent years, it is not necessarily effective in terms of the accuracy of spherical aberration corrections to use the amplitude or jitter to evaluate the quality of the reproduced signal to control the spherical aberration correcting element, and it has thus been desirable to provide an apparatus and method for recording/reproducing optical information that can easily, reliably, and accurately correct spherical aberration.